Carbon blacks are generally produced in a furnace-type reactor by pyrolyzing a hydrocarbon feedstock with hot combustion gases to produce combustion products containing particulate carbon black. A variety of methods for producing carbon blacks are generally known.
In one type of a furnace carbon black reactor, such as shown in U.S. Pat. No. 3,401,020 to Kester et al., or U.S. Pat. No. 2,785,964 to Pollock, hereinafter "Kester" and "Pollock" respectively, a fuel, preferably hydrocarbonaceous, and an oxidant, preferably air, are injected into a first zone and react to form hot combustion gases. A hydrocarbon feedstock in either gaseous, vapor or liquid form is also injected into the first zone whereupon pyrolysis of the hydrocarbon feedstock commences. In this instance, pyrolysis refers to the thermal decomposition of a hydrocarbon. The resulting combustion gas mixture, in which pyrolysis is occurring, then passes into a reaction zone where completion of the carbon black forming reactions occurs.
In another type of furnace black reactor, a liquid or gaseous fuel is reacted with an oxidant, preferably air, in the first zone to form hot combustion gases. These hot combustion gases pass from the first zone, downstream though the reactor, into a reaction zone and beyond. To produce carbon black, a hydrocarbonaceous feedstock is injected at one or more points into the path of the hot combustion gas stream. The hydrocarbonaceous feedstock may be liquid, gas or vapor, and may be the same or different from the fuel utilized to form the combustion gas stream. Generally the hydrocarbonaceous feedstock is a hydrocarbon oil or natural gas. However, other hydrocarbonaceous feedstocks such as acetylene are known in the art. The first (or combustion) zone and the reaction zone may be divided by a choke, or zone of restricted diameter, which is smaller in cross section than the combustion zone or the reaction zone. The feedstock may be injected into the path of the hot combustion gases upstream of, downstream of, and/or in the restricted diameter zone. Furnace carbon black reactors of this type are generally described in U.S. Pat. Reissue No. 28,974 and U.S. Pat. No. 3,922,355.
In both types of processes and reactors described above, and in other generally known reactors and processes, the hot combustion gases are at a temperature sufficient to effect pyrolysis of the hydrocarbonaceous feedstock injected into the combustion gas stream. In one type of reactor, such as disclosed in Kester, feedstock is injected, at one or more points, into the same zone where combustion gases are being formed. In other type reactors or processes the injection of the feedstock occurs, at one or more points, after the combustion gas stream has been formed.
In either type of reactor, following the point of feedstock introduction, the feedstock is mixed, atomized and vaporized into the combustion gas stream. The mixture of combustion gases and vaporized feedstock then enters the primary reaction zone. The term, "primary reaction zone", refers to that zone in the process where the vaporized hydrocarbon feedstock is converted to carbon black primary particles and aggregates. The residence time of the feedstock, combustion gases, and carbon blacks in the primary reaction zone of the reactor is sufficient, and under conditions suitable, to allow the formation of carbon blacks. A secondary reaction zone may or may not exist in the reactor downstream of the primary reaction zone. In those cases where the secondary reaction zone exists, the term "secondary reaction zone" refers to that portion of the reactor where surface modification of the carbon blacks formed in the primary reaction zone takes place. The mixture of combustion gases and carbon blacks in the secondary reaction zone of the reactor is hereinafter referred to, throughout the application, as "the effluent". After carbon blacks having the desired properties are formed, the temperature of the effluent is lowered to stop the major reactions. This lowering of temperature of the effluent to stop the major reactions may be accomplished by any known manner, such as by injecting a quenching fluid, through a quench, into the effluent. As is generally known to those of ordinary skill in the art, the major reactions are stopped when the desired carbon blacks have been produced in the reactor, as is determined by sampling the carbon black and testing for analytical properties. After the reactions have been stopped and the effluent sufficiently cooled by any known means, the effluent generally passes through a bag filter, or other separation system to collect the carbon black.
Although two general types of furnace carbon black reactors and processes have been described, it should be understood that the present invention can be used in any other furnace carbon black reactor or process in which carbon black is produced by pyrolysis and/or incomplete combustion of hydrocarbons. This process differs from prior technology in that an oxidant-containing stream is introduced into the secondary reaction zone in order to accelerate and promote surface modifying reactions in the secondary reaction zone only. Throughout this application, the term "oxidant-containing stream" refers to any stream which contains an oxidizing agent. Preferably, `oxidant-containing stream` refers to air, oxygen-enriched air, combustion products of hydrocarbon fuels and air and/or oxygen, or mixtures of these streams. This oxidant-containing stream does not interfere with reactions or processes occurring in the primary reaction zone in which the carbon black primary particles and aggregates are formed.
In the prior art, there are references such as U.S. Pat. Nos. 3,607,058; 3,761,577; and 3,887,690 which describe the introduction of secondary heat into a carbon black reactor. These references differ from the present invention in that a minimum residence time, determined by the temperature increase of the effluent after addition of an oxidant-containing stream to the secondary reaction zone, is required to achieve the benefits of the present invention. No residence time or minimum temperature rise after addition of secondary heat is specified in the prior art references. From the examples cited in U.S. Pat. No. 3,887,690, the analytical properties of the carbon blacks, particularly the fact that the Nitrogen Surface Area is greater than the Iodine Adsorption Number in all cases, indicate that the minimum residence time requirement of the present invention has not been achieved.
Carbon blacks may be utilized as pigments, fillers, reinforcing agents and for a variety of other applications. Carbon blacks are widely utilized as fillers and reinforcing pigments in the compounding and preparation of rubber compositions and plastic compositions. Carbon blacks are generally characterized on the basis of their properties including, but not limited to, their surface areas, surface chemistry, aggregate sizes and particle sizes. The properties of carbon blacks are analytically determined by tests known to the art, including iodine adsorption surface area (I.sub.2 No), nitrogen adsorption surface area (N.sub.2 SA), dibutyl phthalate adsorption (DBP), dibutyl phthalate adsorption of the crushed carbon black (CDBP), cetyl-trimethyl ammonium bromide absorption value (CTAB), Tint value (TINT), Dmode and .DELTA.D50.
It is generally understood that the properties of a carbon black affect the properties of rubber or plastic compositions containing the carbon black. For example, the introduction of carbon black into a rubber or plastic composition during formation of the composition will generally affect the viscosity of the rubber or plastic composition. Increasing the carbon black loading in a rubber or plastic composition normally increases the viscosity of the composition at a given temperature. Lower viscosity rubber or plastic compositions are advantageous because they are more easily processed.
In addition to a variety of other uses, such as tire, hoses, belts, and plastics, carbon blacks are generally utilized in compositions intended for use as semi-conductive shielding compounds for electric power cables. Electric power cables generally consist of electrically conductive wires surrounded by a dielectric insulating material which prevents escape of electricity to the environment. These semi-conductive shielding compounds are critical for long cable life because they reduce the electrical stress between the conductive and insulating portions of the cable. It is generally desirable in the production of semi-conductive shielding compounds for electrical power cables to use carbon blacks which impart electrical conductivity to the shielding compound. In evaluating the conductivity of a plastic composition, the composition's resistivity is generally measured. However, it is widely understood that conductivity is simply the inverse of resistivity. The required degree of conductivity in the shielding compound can be achieved by increasing the loading of carbon black in the composition, but this also increases the compound viscosity. Therefore, it is apparent that it is advantageous to use a carbon black that imparts the required degree of conductivity while minimizing the compound viscosity. The advantage of optimizing this combination of properties is not limited to semi-conductive shielding materials for power cables.
The ASTM 300% modulus of a rubber compound is a measure of the compound's stress-strain relationship. ASTM Test D3192 describes the evaluation of modulus for a rubber compound. Carbon black specifications are often set based on the ability of the carbon black to impart a range of modulus values to a compound within certain narrow tolerances. It is advantageous to have a process that enables the carbon black producer to manipulate a composition's modulus for a given type of carbon black. Additionally, for certain applications, such as off-the-road automobile tires, a carbon black that imparts low modulus to rubber compounds is considered advantageous.
The Compound Moisture Absorption (CMA) property of a rubber or plastic composition relates to the composition's tendency to absorb moisture. It is generally desirable, for most applications, to have rubber or plastic compositions that do not absorb moisture. Therefore, it is advantageous to have a carbon black that, when incorporated into rubber or plastic compositions, results in a lower CMA for the composition. Lower CMA values are generally understood to relate to lower absorption of moisture.
As will be understood from the foregoing discussion, it would be advantageous to have a process for producing carbon blacks that impart improved conductivity to plastic or rubber compositions. It would be further advantageous to have class of carbon blacks that impart improved conductivity and lower viscosity to plastic or rubber compositions.
It would also be advantageous to have a process for producing carbon blacks that impart lower modulus to rubber compositions, lower CMA to plastic or rubber compositions and lower viscosity to plastic or rubber compositions.
The process of the present invention achieves the afore-mentioned advantages in addition to other advantages that will become apparent to those of ordinary skill in the art from the following description. Similarly the carbon blacks of the present invention achieve the afore-mentioned advantages and other advantages that will become apparent from the following description.